Frequently, in aerospace applications, it is desired to illuminate a distant target with a highly collimated beam, such as a laser beam. During the time that the target is illuminated, it may be desirable to change the divergence of the laser beam (and hence the size of the beam on the target). One reason for this may be that the target is changing its position, and hence its distance, to the platform that contains the laser. Therefore, if the divergence is changed, the overall power illuminating the target may be preserved, or changed. For example, if the target is receding from the laser source, it may be desirable to reduce the divergence of the transmitted laser beam in order to maintain, or even increase the power incident on the target.
Furthermore, there are some circumstances where it is desired to change the size of the transmitted beam very quickly. In cases where rapid changes in size are required of a beam transmitted over very large distances, there is a heightened need to make these optical changes while keeping any change in the pointing of the transmitted illumination source as minimal as possible, since even relatively minor changes in beam direction at the source can result in significant deflections of the beam by the time it reaches the target.
In order for conventional optical systems to change the divergence of a transmitted beam, a conventional lens called a “zoom lens,” such as an afocal zoom lens. Afocal zoom lenses are used in this example because they are particularly suitable for use in changing the divergence of collimated light sources, such as that used in a laser.
In a conventional zoom lens assembly, there are lenses and lens groups that move on mechanical stages, or platforms, along the longitudinal axis of the lens. The mechanical precision, stability, and speed of these motion devices must be exorbitant in order to change the zoom magnification quickly and maintain the pointing direction precisely. This is mainly because the powered optical elements of such an assembly are moving with respect to each other and, to preserve the pointing direction, the positions of their relative centers must be preserved along the original optical axis to the same degree of precision of the mechanical assembly tolerances of the lenses.
In the context of this disclosure, powered optical elements should be understood to refer to curved optical elements that change the direction of light passing therethrough. In short, such elements are considered to have “optical power.” As an example, a lens which is of a plano-convex shape has positive optical power (i.e. it focuses light incident thereon), while a lens with a plano-concave shape has a negative optical power (it causes a divergence of light incident thereon).
Conversely, unpowered optical elements, as used in this disclosure, should be understood to refer to relatively flat optical elements that do not change the direction of light passing therethrough. In short, unpowered optical elements have no optical power, i.e. no optical bending power. The “unpowered” term only refers to the ability of the optical element to bend light that passes through it. A slab of glass with plano sides is an example of an unpowered optic.
Importantly, the terms “powered” and “unpowered,” as they are used in the present disclosure, are not meant to convey any information regarding whether or not the optical elements are rotated, translated, or otherwise moved by a motor or by a user.
There are many examples of zoom lenses described in public resources, such as journal publications and patents. For example, the following 3 US patents describe zoom lenses and zoom mechanisms: U.S. Pat. No. 3,825,315 (1974), Zoom Lens Optical System for Infrared Wavelengths; U.S. Pat. No. 5,587,843 (1996), Zoom Lens Mechanism; and U.S. Pat. No. 4,885,600 (1989), Zoom mechanism for a zoom lens in cameras and the like. These patents describe apparatuses that are typical of all zoom lenses in that they employ various mechanical devices to move the powered optical elements with respect to each other along a common axis. Due to this basic similarity in their design, they all suffer from the difficulties and limitations described above.
The present disclosure solves these problems by describing a device and a method that allow for the rapid change of zoom states, without impacting the mechanical pointing precision of the powered optical elements.